Controlled circulation stowable rotor for v/stol aircraft

ABSTRACT

A stoppable helicopter-type rotor is provided with means to blow air angularly outward toward the leading and trailing edges, above and below each rotor blade. The rotor includes means to stop and fold the blades and stow the folded structure in an enclosure in an aircraft, the air blowing being used during transition between vertical and horizontal flight modes to spoil the lifting effect of the blades during stopping of rotation of the rotor and so eliminate the blade-bending loads which are a major problem with stoppable rotors.

United States Patent [72] Inventor Peter F. Girard La Mesa, Calif. [21]Appl. No. 875,226 [22] Filed Nov. 10, 1969 [45] Patented Oct. 12, 1971[73] Assignee Ryan Aeronautical Company San Diego, Calif.

[54] CONTROLLED CIRCULATION STOWABLE ROTOR FOR V/STOL AIRCRAFT 16Claims, 12 Drawing Figs.

52 US. Cl 244/7, 244/42, 244/49, 416/90 [51] Int. Cl B64c 27/22 [50]Field of Search 244/7, 42, 41,65, 49;416/90, 142, 20

[56] References Cited UNITED STATES PATENTS 2,084,464 6/1937 Stalker416/90 2,925,129 2/1960 Yuan et al. 416/90 X 3,446,288 5/ 1969 Yuan416/90 X 3,515,500 6/1970 Nachod 244/7 X Primary ExaminerMilt0n BuchlerAssistant Examiner-C. A. Rutledge Attorneys-Carl R. Brown, Stephen L.King and Kenneth W.

Mateer PATENTED um I 2 I871 SHEET 10F 5 INVENTOR. PETER F. GIRARD ATTO RN EY PATENTEI] 0m 2197:

SHEET 2 [IF 5 INVENTOR. PETER F. GIRARD ATTORNEY PATENTEU BET I 2 IQYISHEET 3 OF 5 Fig. 6

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I0 I Fig. II

ROTOR CLUTCH 244 ROTATING BRAKE 8 INDEX STOPPED- 4 MR 252 BLOWING H/zsoLOCKING BLADE FOLDED PINS FOLDING ROTOR DOOR STOWED RETRACTION CONTROL246 INVENTOR.

Fig, [2 PETER F. GIRARD ATTORNEY BACKGROUND OF THE INVENTION In order tocombine the vertical flight capabilities of a helicopter and the highspeed of conventional aircraft, various compound aircraft with stoppablerotors and fixed wings have been proposed and tested. To reduce oreliminate the drag of the nonfunctioning rotor in high-speed flight,means have been devised to fold and stow the rotor within the aircraft.One very serious problem in such an aircraft is the destructive loadswhich occur in the rotor blades during stopping of the rotor whentransitioning from rotary wing to stowed rotor flight. When the rotor isrotating near design r.p.m., the blades are effectively stiffened bycentrifugal force and are resistant to loads imposed by cyclicallychanging airspeed or gust conditions. In an unpowered mode, at very lowand zero r.p.m. the blades are subjected to very large bending momentsand other loads due to airspeed changes between the advancing retreatingsectors of rotation relative to flight direction, gusts or otherspurious disturbances dynamic resonance at certain rotational speeds,and other factors. To withstand such .loads the rotor structure must beunnecessarily heavy, or complex automatic stabilization means must besued to minimize these loads while the rotor is being stopped and foldedfor stowage.

SUMMARY OF THE INVENTION In the rotor described herein, the aerodynamiclift of the blades is destroyed during transitional flight, soeliminating the major cause of blade-bending loads. This is accomplishedby blowing air through spanwise rows of small holesalong each rotorblade, the air being directed angularly outward toward the leading andtrailing edges, above and below the blade. The effect is essentiallyopposite to the well-known boundary layer control techniques, which areconcerned with enhancing lifting flow. The leading edge flow across andagainst the normal flow, when the blade is advancing inthe generaldirection of flight, disrupts and separates the liftgenerating flow overthe blade airfoil. As forward speed of the aircraft increases, reverseflow occurs over the blades in the retreating sector of rotation and thetrailing edge blowing spoils by lift which might .be produced. Theeffect occurs at any blade pitch angles and cyclic control of the bladesduring transition is unnecessary. Airflow is controlled by valves whichprevent undesirable blowing during powered rotor operation, and areautomatically operated when air blowing is intentionally started.

The air-blowing system is readily adaptable to a foldable rotor in avariety of aircraft types, using different types of propulsive power.There is no interference with normal control and the transitionoperation requires a minimum of attention on the part of the pilot.

Other objects and many advantages of this invention will become moreapparent upon a reading of the following detailed description and anexamination of the drawings, wherein like reference numerals designatelike parts throughout and in which:

BRIEF DESCRIPTION OF THE DRAWING FIG. I is side elevation view of atypical aircraft with the controlled circulator rotor extended.

FIG. 2 is a view similar to a portion of FIG. I, with the rotor stowed.

FIG. 3 is an enlarged top plan view of the inboard end of one rotorblade.

FIG. 4 is a sectional view taken on line 44 of FIG. 3.

FIG. 5 is an enlarged sectional view taken on line 5-5 of FIG. 3.

FIG. 6 is atop plan view of the rotor head assembly.

FIG. 7 is an front elevation view, partially cut away, of the rotorsupport and drive structure.

FIG. 8 is a side elevation view as taken from the right-hand side ofFIG. 7.

FIG. 9 is an enlarged sectional view taken on line 99 of FIG. 8.

FIG. 10 is an enlarged front elevation view of the rotor head as shownin FIG. 6.

FIG. 11 is an enlarged sectional view taken on line -11 of FIG. 2.

FIG. 12 is a diagram of the basic control functions of the system.

DESCRIPTION OF THE PREFERRED EMBODIMENT The aircraft shown in FIG. 1 isa twin-engined transport type with a fuselage 10, a fixed wing 12carrying engines 14, and a T-tail assembly 16 on which is mounted anantitorque rotor 18. In the top of the fuselage I0 is a recessed boxsection 20 in which the rotor unit 22 is mounted, a channel 24 extendinglongitudinally rearwardly from the box section to hold the folded rotorblades, as in FIG. 2. The box section and channel are normally coveredby hinged doors 26, which are open during rotor operation. It should beunderstood that the aircraft shown is merely an example and that theairframe and propulsion means can vary considerably according to theintended function of the aircraft. The rotor may be independentlypowered, but is shown in this instance as driven. by a drive shaft 38from a power takeoff gearbox 30 coupled to one or both engines 14, aclutch 32 being installed in the drive shaft for control of rotor power.

A three-bladed rigid-type rotor is shown, but any practical number ofblades may be used and the system is equally adaptable to an articulatedrotor with several degrees of freedom in the blades. Rotor unit 22 has ahead 34 mounted on a shaft 36, the head having three radial arms 38, 40and 52 to which the rotor blades are attached. For descriptive purposesthe blades will be referred to as a left blade 44, right blade 46 andrear blade 48, with respect to the stopped position of the rotor priorto folding as in FIG. 6. All three blades are essentially the same andthe rear blade only will be described in detail, the elements beingcorrespondingly numbered in the other blades.

The blade structure can vary and is shown in FIG. 5 as comprising aD-box leading edge 50 with a trailing portion formed by top and bottomskins 52 and 54 over a spanwise rear spar 56, the contours being held bya foam filling 58. In the forward portion of leading edge 50 is aspanwise cylindrical sleeve valve 60 which is axially rotatable, thesleeve having diametrically opposed ports 62 spaced along its length. Inone position of sleeve valve 60, the ports 62 register with small outletholes 64 extending angularly outwardly and forwardly through the top andbottom of the leading edge, the sleeve valve serving as anair-conducting passage communicating with all of the outlets. Within therear spar 56 is a similar sleeve valve 66, with ports 68 which registerwith outlet holes 70 extending angularly outwardly and rearwardlythrough skins 52 and 54. At the inboard ends the sleeve valves are eachfitted with a radial arm 72 and the arms are coupled by a connecting rod74 to move together and rotate the sleeve valves. Connecting rod 74 isthe shaft of an actuating cylinder 76 mounted on the inboard end of therotor blade, and is fitted with a piston 78 sliding in the cylinder. Aspring 80 biases the piston 78 to the rear of the cylinder to holdsleeve valves 60 and 66 in closed position, as in full line in FIG. 4,in which position the ports in the sleeves are out of register with therespective outlet holes and no outlet flow can occur. This valve closingis necessary to prevent undesirable flow through the outlets due tocentrifugal pumping in powered flight. When piston 78 is driven forward,the sleeve valves are rotated to the open position shown in FIG. 5 andin broken line in FIG. 4.

At the inboard end of the blade, the leading edge has an air inlet 82from which a chordwise duct 84 communicates with both sleeve valves toprovide air thereto. From duct 84 a small bleed tube 86 leads tocylinder 76 behind the piston 78, so that when air pressure builds up inthe duct, the piston is pushed forward and the sleeve valves are opened,making the valve action automatic. When air pressure is shut off thevalves are automatically closed by spring 80, the spring being selectedaccording to the actuating pressure required.

Fixed to the root end of the rotor blade is a support fitting 88 whichis mounted on an attachment fork 91) and is rotatable on the fork abouta spanwise axis for blade pitch control. EAch fork 90 has a pair ofenlarged bosslike lugs 92 and 94 for strength of attachment. Projectingfrom support fitting 88 is a pitch control arm 96 for connection toconventional control means, hereinafter described. Thus far the bladesare similar, it is in their attachment to the rotor head that thedifference lies.

Rear blade 48 is fixed and is secured to arm 42 by suitable bolts 98 orthe like, through lugs 92 and 94 and corresponding lug elements on thearm. The other two blades are hinged to swing reanivardly, substantiallyin the plane of the rotor, to extend in overlapping parallel relationwith blade 48, as in the broken line positionsin FIG. 6.

Left blade 44 is attached to a rear lug 100 on arm 38 by a hinge pin 102through lug 92, as in FIG. 10. Right blade 46 is similarly attached to arear lug 104 on arm 40 by a hinge pin 106, which passes through lug 94of the right blade since the blades are handed and swing in oppositedirections. Lug 94 of left blade 44 fits between lugs 108 and 110 at thefront of arm 38 and is held by a lock pin 112. Lug 92 of right blade 46similarly fits between lugs 114 and 116 at the front of arm 40 and isheld by a lockpin 118. When lockpins 112 and 118 are partiallywithdrawn, clear of the respective attachments fork lugs, the blades canswing about their hinge pins.

Various means can be used to operate the lockpins, a mechanical linkagebeing shown as an example in FIG. 10. A linear actuator 120 has anactuating rod 122 from which a pair of links 124 are pivotally connectedto the inner ends of arms 126, the outer ends of the arms beingpivotally attached to the upper ends of the lockpins. Adjacent theirinner ends the arms 126 are pivotally attached to support bars 128which, in turn, are pivotally mounted on the rotor head 34. Whenactuating rod 122 is pulled down the arms 126 hinge on support bars 128and lockpins 112 and 118 are pulled up, as in broken line in FIG. 10.The support bars 128 and links 124 pivot to accommodate the motion andallow parallel action of the lockpins.

Various means can also be used to swing the rotor blades betweenextended and folded positions. FIG. 6 shows a gear mechanism in which amotor 130 has a worm 132, or similar slow-motion means, driving a ringgear 134 coaxial with the rotor head axis. Right blade 46 has a sectorgear 136 fixed in relation to attachment fork 90 and coupled to ringgear 134 by an idler gear 138. Left blade 44 has a sector gear 140flexed to its attachment fork 90 and coupled to ring gear 134 by doubleidlers 142 and 144, to obtain opposite motion to the right blade. Bothblades are thus folded and extended in proper synchronization.

On top of rotor head 34 is a plenum chamber 146, from which a flexibleair hose 148 extends to inlet 82 of the rear blade 48. From plenumchamber 146 a pair of rigid air ducts 150 extend to couplings 152mounted on and coaxial with the hinge pins 102 and 106. Flexible airhoses 154 lead from the couplings 152 to inlets 82 of the left and rightblades. The flexible hoses allow for blade pitch change motions and canaccommodate the folding motion, or the couplings 152 may be mounted torotate with the blades, as indicated inF-IG. 6. Air is supplied toplenum chamber 146 through hollow shaft 36, by any suitable connection.Rotor shaft 36 is held in a drive unit 156 containing a planetary orother gear drive 158 of conventional type and a large bevel gear 160 towhich driving power is applied. To provide for retraction of the rotorunit into the aircraft, drive unit 156 is pivotally mounted betweentrunnions 162 and 164 at the upper end of a yoke 166. The lower end ofthe yoke 166 is pivotally mounted between trun nions 168 and 170,secured to fixed aircraft structure 172, in box section 20. The axes ofthe two sets of trunnions are parallel to each other and to the spanwiseaxis of the aircraft, so

that the drive unit and rotor can swing forward and down into theaircraft.

To connect driving power to gear through the pivotal mounting of thedrive unit, drive shaft 28 has a bevel gear 174 engaging a similar gear176 on a shaft 178 coaxially mounted through trunnion 170. Inside theyoke 166, shaft 178 has a bevel gear 180 engaging a similar gear 182 ona shaft 184 extending into trunnion 164, and having a further bevel gear186 thereon. The last-mentioned gear engages a similar gear 188 on ashaft 190 mounted coaxially in trunnion 164 having a final gear 192 indriving connection with gear 160. The arrangement is merely an exampleand other drive couplings may be equally suitable.

Retraction and extension of the rotor assembly is accomplished by meansof a stowage motor 194, mounted in the aircraft, with a worm 196engaging a worm gear sector I98 fixed on yoke 166 coaxial with trunnion170, as in FIG. 8. In order to hold the rotor shaft in a near uprightposition during retraction, a stabilizing link 200 is pivotallyconnected between a bracket 202 on drive unit 156 and a bracket 204 onfixed structure 172. In actual practice, with the rigid-type rotorshown, a slight rearward inclination of the rotor axis might benecessary in the stowed position, so that the rotor blades can beconveniently enclosed.

Fixed at a convenient position on rotor shaft 36 is a braking disc 206which is retarded by a brake unit 208 having one or more pressure pads210, in the manner of the well-known disc brake apparatus, to facilitatestopping of the rotor. To ensure proper indexing of the rotor blades forstowing, the braking disc 206 has an indexing notch 212 in itsperiphery. An actuator 214 mounted on brake unit 208 has an indexingroller 216 which is biased against the edge of braking disc 206 anddrops into notch 212 to lock the rotor in place, with rear blade 48along or parallel to the longitudinal axis of the aircraft.

The rotor is controlled by conventional means, such as a swashplate 218concentric with shaft 36 and having link rods 220 coupled to the pitchcontrol arms 96. The arrangement suitable for a folding rotor is wellknown and has not been shown in detail. To accommodate the stowageaction of the rotor, the controls must be adapted in a suitable manner,as in FIG. 7. Yoke 166 has a bracket 222 carrying a pivoted arm 2% whoseends are substantially in axial alignment with trunnions 162 and 168. Aconnecting rod 226 from the pilots controls is connected to the lowerend of arm 224 by a rotatable coupling 228. From the upper end of arm224, a link 230 extends substantially parallel to the axis of trunnion162 to a bellcrank 232, pivotally mounted on a bracket 234 on drive unit156. Link 230 also has rotatable ends to allow out of plane notionbetween arm 224 and bellcrank 232. In actual practice several arms 224and several bellcranks 232 would be grouped together as closely aspossible to the respective trunnion axes, for the various cyclic andcollective pitch controls. From the bellcrank shown, motion istransferred to swashplate 218 through a rocker arm 236 and 238 and 240.A further link 242 is coupled to the swashplate from a concealedbellcrank, as examples of typical connections. The specific arrangementwill depend on the particular aircraft and the rotor controls required.

FIG. 12 shows the order in which the various actions occur and isintended to indicate only the related functions, not a control system. Acontrol lever 244 is movable to four basic positions indicating thecondition of the rotor and is shown as having an arm 246 which actuatesswitches 248, 250 and 252, the switches being assumed to reverse certainactions as they are moved from one position to the other.

For a vertical takeoff the rotor is driven and operated in the manner ofa helicopter. The aircraft is then driven forward until sufficient speedis built up for the fixed wing 12 to develop lift to support theaircraft, the rotor being gradually unloaded as its lift becomesunnecessary. With the aircraft in sustained forward flight the controllever 244 is moved from ROTATING to STOPPED position, which actiondisengages clutch 32, begins to operate braking unit 208 and starts theair-blowing action. A compressor 254 driven by any suitable meanssupplies the compressed air for the system, and is provided with aconventional type of shutoff valve 256. As air pressure builds up thevalves 60 and 66 open and air is ejected from outlets 64 and 70. Thecontrolled circulation of air prevents the rotor blades from developinglift and the rotor can be brought smoothly to a stop. Wind tunnel testshave shown that a very moderate airflow in the manner shown has apronounced efl'ect on lift, the virtual elimination of lift being quitefeasible. In the event that indexing does not occur, a clutch 32 can bemomentarily engaged by an override control 258 to rotate the rotor toindexed position.

With the rotor stopped in proper alignment the control lever is moved toFOLDED position, which causes operation of actuator 120 to withdraw lockpins 112 and 118 and then starts motor 130 to fold the blades. In thefolded position the blades overlap at a positive pitch angle, in themanner shown in FIG. 11. Control lever 244 is then moved to STOWEDposition, which causes operation of stowage motor 194 to retract therotor assembly. At the same time the air blowing is shut off and, whenthe rotor is fully retracted, doors 26 are closed. In the stowedposition the rotor pitch control system is disconnected from the flightcontrols, suitable means being well known.

To return to vertical flight mode for landing, the sequence ofoperations is reversed, the doors being opened and the rotor assemblyraised by moving the control lever to FO DED position, which reversesswitch 248. Air blowing is also started at this time prior to extendingthe rotor. The control lever is then moved to STOPPED position, whichopens the hinged blades and inserts the lock pins. Moving the centrallever from STOPPED to ROTATING POSITION releases the indexing and brakemeans and engages the clutch to apply power to the rotor. Air blowing ismaintained as the rotor builds up speed and is shutoff when the rotorreaches a predetermined rotational speed at which the blades developstable lift.

The various switching and sequencing functions are all controlled byconventional means, the nature of which will depend on the types ofmotors and actuators and the services available in the aircraft. Due tothe stability afforded by the lift spoiling during transition, the pilotcan maintain stable flight without the need for control compensations.The system is adaptable to manual control through all stagesor fullyautomatic sequenced operation.

Having described my invention, Inow claim.

I. In an aircraft having fixed lifting surfaces and forward flightpropulsion means,

a selectively driven rotor mounted on the aircraft and having at leastone lifting blade,

substantially spanwise air-conducting passage means in said blade,

a plurality of spanwise spaced outlet holes in said blade communicatingwith said passage and extending angularly outwardly through at least theupper surface of the blade adjacent the leading and trailing edgesthereof, to direct air angularly across the normal flow over the bladeand spoil the lift thereof,

and means for connecting said passage to a source of compressed air.

2. An aircraft according to claim I and including a further plurality ofspanwise spaced outlet holes communicating with said passage means andextending angularly outwardly through the lower surface of the bladeadjacent and toward the leading and trailing edges thereof,respectively, to direct lift spoiling flow outwardly from the blade.

3. An aircraft according to claim 2 and including valve means in saidair-conducting passage means, selectively operable to open and closesaid outlet holes.

4. An aircraft according to claim 3 and including actuating meansconnected to said valve means, said actuating means being responsive toan increased air pressure in said passage means to open said outletholes.

5. In an aircraft having fixed lifting surfaces and forward flightropulsion means,

a se ectively driven rotor mounted on the aircraft, said rotor having ahead with a plurality of lifting blades thereon,

certain of said blades being hinged on said head to swing substantiallyin the plane of the rotor into a generally parallel compact group ofblades,

mounting means securing said rotor to the aircraft, said mounting meansbeing foldable to retract the rotor into the aircraft,

each of said blades having lift spoiling means therein, and

means to actuate the lift-spoiling means during transition betweenvertical and horizontal flight.

6. An aircraft according to claim 5, wherein said lift-spoiling meanscomprises a plurality of spanwise spaced outlet holes opening angularlyoutwardly from the blade adjacent and toward the leading and trailingedges thereof in at least the upper surface of the blade, and a sourceof compressed air communicating with said outlet holes.

7. An aircraft according to claim 6, wherein said outlet holes are inthe upper and lower surfaces of the blade.

8. An aircraft according to claim 5, wherein said lift-spoiling meanscomprises a plurality of spanwise spaced outlet holes opening angularlyoutwardly and forwardly from the upper surface of the blade adjacent theleading edge thereof, a plurality of spanwise spaced outlet holesopening angularly outwardly and rearwardly from the upper surface of theblade adjacent the trailing edge thereof, air conducting passages insaid blade communicating with said outer holes, and a source ofcompressed air connected to said passages.

9. An aircraft according to claim 9 and including valve means in saidpassages for selective opening and closing of said outlet holes.

10. An aircraft according to claim 9 and including air pressureresponsive actuating means connected to said valve means to open saidoutlet holes upon a predetermined increase in air pressure in saidpassages.

11. An aircraft according to claim 10, wherein said actuating means isbiased to move said valve means to the closed position of said outletholes when air pressure in said passages is below a predeterminedpressure.

12. An aircraft according to claim 8 and including a further pluralityof spanwise spaced outlet holes opening angularly outwardly andforwardly from the lower surface of the blade adjacent the leading edgethereof.

13. An aircraft according to claim 12 and including a further pluralityof spanwise spaced holes and opening angularly outwardly and rearwardlyfrom the lower surface of the blade adjacent the trailing edge thereof.

14. An aircraft according to claim 13, wherein said air-conductingpassages comprise a substantially cylindrical spanwise passage in theleading edge portion of the blade communicating with the upper and lowersurface outlet holes therein, and a substantially cylindrical spanwisepassage in the trailing edge portion of the blade communicating with theupper and lower surface outlet holes therein.

15. An aircraft according to claim 14 and including valve meanscomprising sleeve valves axially rotatable in said passages and havingports registrable with said outlet holes in one position of the valves.

16. An aircraft according to claim 15 and including air pressureresponsive actuating means connected to said sleeve valves to rotate thesleeve valves simultaneously and open said outlet holes upon apredetermined increase in air pressure in said passages.

1. In an aircraft having fixed lifting surfaces and forward flightpropulsion means, a selectively driven rotor mounted on the aircraft andhaving at least one lifting blade, substantially spanwise air-conductingpassage means in said blade, a plurality of spanwise spaced outlet holesin said blade communicating with said passage and extending angularlyoutwardly through at least the upper surface of the blade adjacent theleading and trailing edges thereof, to direct air angularly across thenormal flow over the blade and spoil the lift thereof, and means forconnecting said passage to a source of compressed air.
 2. An aircraftaccording to claim 1 and including a further plurality of spanwisespaced outlet holes communicating with said passage means and extendingangularly outwardly through the lower surface of the blade adjacent andtoward the leading and trailing edges thereof, respectively, to directlift spoiling flow outwardly from the blade.
 3. An aircraft according toclaim 2 and including valve means in said air-conducting passage means,selectively operable to open and close said outlet holes.
 4. An aircraftaccording to claim 3 and including actuating means connected to saidvalve means, said actuating means being responsive to an increased airpressure in said passage means to open said outlet holes.
 5. In anaircraft having fixed lifting surfaces and forward flight propulsionmeans, a selectively driven rotor mounted on the aircraft, said rotorhaving a head with a plurality of lifting blades thereon, certain ofsaid blades being hinged on said head to swing substantially in theplane of the rotor into a generally parallel compact group of blades,mounting means securing said rotor to the aircraft, said mounting meansbeing foldable to retract the rotor into the aircraft, each of saidblades having lift spoiling means therein, and means to actuate thelift-spoiling means during transition between vertical and horizontalflight.
 6. An aircraft according to claim 5, wherein said lift-spoilingmeans comprises a plurality of spanwise spaced outlet holes openingangularly outwardly from the blade adjacent and toward the leading andtrailing edges thereof in at least the upper surface of the blade, and asource of compressed air communicating with said outlet holes.
 7. Anaircraft according to claim 6, wherein said outlet holes are in theupper and lower surfaces of the blade.
 8. An aircraft according to claim5, wherein said lift-spoiling means comprises a plurality of spanwisespaced outlet holes opening angularly outwardly and forwardly from theupper surface of the blade adjacent the leading edge thereof, aplurality of spanwise spaced outlet holes opening angularly outwardlyand rearwardly from the upper surface of the blade adjacent the trailingedge thereof, air conducting passages in said blade communicating withsaid outer holes, and a source of compressed air connected to saidpassages.
 9. An aircraft according to claim 9 and including valve meansin said passages for selective opening and closing of said outlet holes.10. An aircraft according to claim 9 and including air pressureresponsive actuating means connected to said valve means to open saidoutlet holes upon a predetermined increase in air pressure in saidpassages.
 11. An aircraft according to claim 10, wherein said actuatingmeans is biased to move said valve means to the closed position of saidoutlet holes when air pressure in said passages is below a predeterminedpressure.
 12. An aircraft according to claim 8 and including a furtherplurality of spanwise spaced outlet holes opening angularly outwardlyand forwardly from the lower surface of the blade adjacent the leadingedge thereof.
 13. An aircraft according to claim 12 and including afurther plurality of spanwise spaced holes and opening angularlyoutwardly and rearwardly from the lower surface of the blade adjacentthe trailing edge thereof.
 14. An aircraft according to claim 13,wherein said air-conducting passages comprise a substantiallycylindrical spanwise passage in the leading edge portion of the bladecommunicating with the upper and lower surface outlet holes therein, anda substantially cylindrical spanwise passage in the trailing edgeportion of the blade communicating with the upper and lower surfaceoutlet holes therein.
 15. An aircraft according to claim 14 andincluding valve means comprising sleeve valves axially rotatable in saidpassages and having ports registrable with said outlet holes in oneposition of the valves.
 16. An aircraft according to claim 15 andincluding air pressure responsive actuating means connected to saidsleeve valves to rotate the sleeve valves simultaneously and open saidoutlet holes upon a predetermined increase in air pressure in saidpassages.